DE112010003918B4 - Detection and detection of a servo pattern - Google Patents

Abstract

A method, servo system, and tape drive for achieving lock on a servo signal are provided. The servo signals of at least one servo element are monitored in order to identify a valid servo signal. If no valid servo signal is recognized, the fine actuator is moved in a first direction until either a valid servo signal is recognized or the limit of the travel path of the fine actuator is reached. If a valid servo signal is recognized, the fine actuator is set to the zero position and the coarse actuator is moved in the first direction. If the limit of the travel path of the fine actuator is reached, the fine actuator is moved to the first position and the coarse actuator is moved in a second direction opposite to the first direction.

Description

TECHNICAL AREA

The invention relates to servo systems for laterally positioning data heads relative to a magnetic tape, and more particularly to detecting and detecting a tracking servo pattern recorded on the magnetic tape.

BACKGROUND

Magnetic tape provides a means for physically storing data whereby the magnetic tape can be archived or stored in storage racks of automatic data storage libraries and accessed as needed. One method of maximizing the amount of data that can be stored is to maximize the number of parallel tracks on the disk, which is usually accomplished by using servo systems that provide tracking and allow very little spacing between tracks.

Magnetic tape data storage tracking servo systems typically include a pre-recorded servo track tracking servo pattern that permits precise positioning of a tape head with servo elements with respect to the servo tracks. The tape head includes one or more read / write elements that are precisely positioned with respect to the servo elements and track data tracks parallel to the servo tracks. In particular, two servo elements mounted on the tape head read the servo pattern and feed the servo signals into a servo control loop.

In high track density tape storage units, a composite actuator can be used in a tracking system. The composite actuator usually includes a coarse actuator and a fine actuator. The fine actuator part has a high bandwidth response and can follow rapid changes in tape guidance. The fine actuator is commonly used for "fine" tracking and allows the tape head to accurately track small changes in the tape position. However, the range of motion of the fine actuator is limited and typically does not span the entire dynamic range of motion required to track a data track over the entire tape length. The coarse actuator is commonly used for long-range movement of the tape head, such as achieving lock on the servo signal and shifting the tape head from one set of servo tracks to another set of servo tracks.

While the coarse actuator is capable of very large strokes, the coarse actuator responds much more slowly.

The tape is usually contained in a cassette with one or two reels, the tape being moved between a supply reel and a take-up reel. The coils usually have an out-of-roundness, as a result of which the band moves laterally during the longitudinal movement. To limit the amplitude of the lateral movement of the tape, tape guides are often provided so that the lateral mobility of the tracking servo system is not exceeded. However, even when using tape guides still occur fast lateral short-term shifts. It can be seen that the occurrence of a lateral momentary shift can occur so quickly and the distances covered can be so great that the servo signal is no longer within the range of the fine actuator. In addition, it should be noted that the coarse actuator may move too slowly to detect the servo signal within a servo lock time threshold. As a result, the reading can be stopped due to an inability to track. Likewise, the writing can be stopped to prevent the overwriting of an adjacent track and to cause no readback error.

As the track densities of tape increase even further, the sideways motion problem also increases with a guided tape path. Therefore, there is a continuing need for improved tape positioning systems to achieve rapid engagement with the servo signal during operation and to provide precise stability and tracking of the tape relative to the tape head.

US 2009/141 405 A1 discloses an actuator assembly for positioning a transducer having a primary and a secondary frame mounted for transverse movement. An encoder is connected to a coarse actuator and a fine actuator. The encoder is positioned by the fine actuator relative to a lateral tape position and if the position exceeds a predetermined range, then the encoder is positioned by the drive of the coarse actuator. The belt positioning gauge measures a lateral tape position of the tape and communicates the position to the actuators via a controller.

US 57 93 573 A discloses an actuator of a high performance tape drive for coarse and fine positioning of tape, a read / write head having at least a pair of cantilever springs supporting a read / write head for controlled movement relative to a movable storage tape.

US 72 80 307 B2 discloses a magnetic tape drive having a compound (coarse / fine) servo actuator that reduces the possibility of servo unit overshoots when a tape cartridge is loaded into the drive, thereby reducing servo failures. The coarse motor is activated while the fine actuator is deactivated to seek a servo signal. When a servo signal is found by the servo elements, the coarse motor is deactivated.

DE 601 05 437 T2 discloses a servo system for laterally positioning magnetic heads with respect to longitudinally defined servo tracks or stripes recorded on a magnetic tape, and more particularly, restoring the lateral position after exiting the lateral position.

SUMMARY

The problem of lateral movement is increasing, as there is a trend towards higher and higher band densities. This is because the tracks become smaller, closer together and the possibility of misalignment increases. Therefore, there is a continuing need for improved tape positioning systems to be fast during operation. Snap to the servo signal and provide precise stability and tracking of the tape relative to the tape head.

Accordingly, embodiments of the present disclosure provide methods, servo systems, and tape drives that use the fine actuator to achieve latching on the servo signal. In one embodiment, the fine actuator is set to a first position. In one embodiment, the first position is the zero position with respect to the coarse actuator. The servo signals of at least one servo element are monitored to detect a valid servo signal. If no valid servo signal is detected, the fine actuator is moved in a first direction until either a valid servo signal is detected or the limit of the travel of the fine actuator is reached.

In one embodiment, in the event that a valid servo signal is detected, the fine actuator is set to the first position and the coarse actuator is moved in the first direction. Furthermore, in one embodiment, in case the limit of the travel of the fine actuator is reached, the fine actuator is set to the first position and the coarse actuator is moved in a second direction opposite to the first direction.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described by way of example with reference to the accompanying drawings, in which:

1 shows a tape drive in which a servo system according to an embodiment of the invention is formed;

2 shows a non-scale block diagram of a tape head and a servo controller according to an embodiment of the invention; and

3 shows a flowchart of a method for detecting a servo pattern according to an embodiment of the invention.

DETAILED DESCRIPTION

1 shows a tape drive 10 such as a magnetic tape drive using a servo system according to an embodiment of the present specification. A magnetic tape 21 is along a tape path from a supply reel 22 in a magnetic cassette 23 to a take-up reel 24 wherein the coils comprise drive coils of a drive system driven by drive motors. The magnetic tape is moved along the tape path in a longitudinal direction over the tape head 25 emotional. The tape head is powered by an actuator 27 a servo system, which may comprise, for example, a compound actuator. The tape head 25 , which may be, for example, a magnetic tape head, may include a plurality of read and write elements and a plurality of servo read elements. The tape can be a variety of servo tracks or tapes 28 which are recorded on the tape in the longitudinal direction of the tape and parallel to the data tracks. The servo read elements (not shown) are part of a tracking servo system for moving the tape head 25 in a lateral direction to follow the lateral movement of the longitudinal tracks when the band 21 is moved in the longitudinal direction, and thereby the tape head 25 to track the data tracks on the data tracks.

The composite actuator 27 has a coarse actuator (in 2 shown below 426 ) such as a stepper motor and a fine actuator (in 2 shown below 420 ) such as a voice coil motor mounted on the coarse actuator. In one embodiment, the fine actuator portion has a high bandwidth response and may follow rapid changes in tape guidance. Of the However, the range of motion of the fine actuator is limited so that it can not cover the entire dynamic range of motion required to track a data track over the entire tape length. In one embodiment, the range of motion (eg, the limit of the travel of the fine actuator) is 900 microns. In one embodiment, the coarse actuator is capable of large strokes and is used to translate the tape head over long lateral distances (eg, over a range of 12 to 8500 microns). Although the coarse actuator is capable of very large strokes, it responds much more slowly than the fine actuator.

An example of a compound actuator is in the U.S. Patent 5,793,573 described. Those skilled in the art will recognize that many different types of composite actuators may be used to form an embodiment of the present invention.

It can be seen that the occurrence of a lateral momentary shift can occur so quickly and the distances covered can be so great that the servo signal is no longer within reach of the fine actuator, and that the coarse actuator moves too slowly to to detect the servo signal within a lock tolerance window. As a result, reading may be stopped due to an inability to track. Likewise, the writing can be stopped to prevent the overwriting of an adjacent track and to cause no readback error.

The tape drive 10 also includes a servo control unit 30 , which provides the electronic modules and the processor to form the servo system for the operation of the composite actuator. The magnetic tape 21 of the present example may be in a tape cassette or cartridge 23 be provided, which is a supply reel 22 or both a supply reel 22 as well as a take-up spool 24 having.

In one embodiment, tape guides reduce 41 . 42 . 43 . 44 an extreme lateral movement of the tape, for example due to roundnesses of the supply reel 22 or the take-up reel 24 at least in terms of the amplitude of the movement of the tape. However, when the tape is wound on a spool, the tape usually undergoes rapid lateral momentary shifts, for example, due to stack displacements or skewed windings in which one tape winding is substantially offset with respect to an adjacent one. Other common causes of fast lateral short-term shifts include: 1) a folded band edge, the band moving against a collar of a band guide and suddenly sliding laterally back down onto the bearing, 2) a damaged band edge, causing the band to edge Belt leaks sideways when in contact with a tape guide; and 3) when the runout or unwind spool is so heavy that the coil's coil hits the edge of the tape. It should be noted that the tape 21 be very thin and have a low lateral edge stiffness.

In one embodiment, the tape guides are 41 . 42 . 43 . 44 to fretless tape guides. While fretless tape guides have the advantage of avoiding damage to the tape edges due to interaction with the fret, they allow more lateral movement of the tape. In some environments with fretless tape guides, side shifts are magnetic tape 21 possible by 1000 microns or more in each direction. This lateral shift makes it difficult for a servo system to detect and maintain a servo signal.

2 Figure 12 is a block diagram of a portion of a tape drive system 400 according to the present embodiment, which is a tape head assembly 411 , a servo control unit 430 and a coarse engine 440 and in which the present invention may be embodied. Also shown is a representative servo pattern 450 in tracks of the tape volume 421 is recorded. The head assembly 411 has a fine actuator 420 with a voice coil motor VCM 414 and a pair of flexures 416 and a tape head 415 on. The tape head 415 has two servo elements 445 whose position is that of two of the servo tracks 450 and one or more read / write elements (not shown) connected between the two servo elements 445 are arranged. - The arrangement of elements on the head 415 in 2 is for illustrative purposes only. It can be seen that further facilities of elements on the head are possible. For example, the invention is applicable to other devices having a head with a single servo element or with more than two servo elements.

The servo control unit 430 has a processor 432 , a store 436 , a digital-to-analog converter (DAC) 434 and a VCM drive 418 (eg a fine servo drive). The processor 432 has an input for receiving position signals from the servo elements 445 be returned, it generates a first output signal through which the coarse motor 440 is controlled, a second output signal through which the DAC 434 an input value is provided, and a third output signal, the the memory 436 provided. In one embodiment, the processor includes 432 a servo logic and programming instructions for achieving a servo lock and tracking the servo pattern 450 ,

The memory 436 stores data and programming for the position control of the servo elements 445 , In one embodiment, the memory may be nonvolatile memory such as ROM, flash memory, magnetic computer memory units (eg, hard disks, floppy disks and magnetic tapes) and optical hard disks. In addition, the memory may be volatile memory such as RAM, DRAM, and SRAM. The memory 436 has an input to receive position data from the processor and an output through which position data is sent to the processor 432 be issued. The DAC 434 has an input that is connected to the output signals of the processor 432 can receive, and an output through which the VCM drive 418 is powered. The VCM drive 418 has an input connected to the output signals of the DAC 434 can receive, and an output that is connected to the VCM 414 of the fine actuator 420 can drive. The VCM 414 deflects the bending elements 416 so make sure they have the servo elements 445 as to the length of the band 421 to move sideways by small distances. The processor 432 , the DAC 434 , the drive 418 , the VCM 414 and the servo elements 445 include the fine servo loop.

The function of the present invention in one embodiment will now be described with reference to the flowchart of FIG 3 described. In step 602 the tape drive starts 10 with the achievement of a latching on the servo signal. In one embodiment, the tape drive begins 10 upon achieving a latch on the servo signal in response to a command to read and / or write to the tape media 421 , In response to a command to read and / or write to the tape media 421 becomes the tape cassette 23 in the tape drive 10 loaded. However, in other embodiments, the cartridge may already be in the tape drive 10 be loaded. In step 604 becomes the fine actuator 420 to a first lateral position with respect to the coarse actuator 426 set. In one embodiment, the first lateral position is the zero position of the fine actuator 420 by setting the output of the DAC equal to 0 (output DAC = 0). In the zero position is the fine actuator 420 of the actuator laterally equidistant from each end of the coarse actuator 426 and can therefore be moved equal to zero position in each direction. It will be apparent to those skilled in the art that the first lateral position is any starting position of the fine actuator with respect to the coarse actuator 426 can act; however, the zero position is preferred.

In step 606 start the servo elements 445 by monitoring servo signals at a first lateral position (eg, zero position where the output DAC = 0) to the servo pattern 450 to recognize.

In step 608 represents the processor 432 fixed, whether the servo elements 445 have detected a valid servo signal. The processor 432 notes that a valid servo signal is present when the predetermined servo lock threshold is reached. The predetermined servo lock threshold can be set by the user, an administrator or the tape drive manufacturer. In one embodiment, the predetermined servo lock threshold is reached when a servo signal is detected in more than fifty percent of the servo samples. For example, if the servo elements have monitored two hundred (200) servo samples and detected one hundred (100) or more servo signals within the two hundred (200) samples, the processor stops 432 determines that the previously set servo lock threshold has been reached and thus a valid servo signal is present. It will be apparent to those skilled in the art that the predetermined servo lock threshold may be different for each user and for different operating conditions. For example, the predetermined servo lock threshold may be changed based on the user's choice of design, system improvements, or risk tolerance. For example, in other embodiments, in the event that a servo signal is detected in more than ten percent of the samples, the processor 432 determine that the previously defined servo lock threshold has been reached and thus a valid servo signal is present. Furthermore, in the event that a servo signal is detected in more than 90 percent of the samples, the processor 432 determine that the previously defined servo lock threshold has been reached and thus a valid servo signal is present.

If the processor 432 in step 608 determines that by the servo elements 445 a valid servo signal is detected, the process goes to step 620 continued. Once in step 620 a valid servo signal is detected within the predetermined servo lock threshold, the processor locks 432 the servo system 400 to the current lateral position of the tape head servo elements 445 a step 620 ). As will be apparent to those skilled in the art, the servo control unit performs 430 performs the normal tracking function of tracking the servo signal at the desired lateral position, enabling reading and / or writing of data to the associated data tracks.

If, however, the processor 432 in step 608 determines that no valid servo signal is detected, becomes the fine actuator 420 by a step size X in a first direction with respect to the coarse actuator 426 moved (step 610 ). If the fine actuator 420 by a step size X in a first direction with respect to the coarse actuator 26 is moved, the output is DAC = DAC + Y, where Y stands for the DAC value, which is the fine actuator 420 moved by one increment X in the first direction. In one embodiment, the step size X may be the distance between a servo band (eg, 200 micrometers). However, it will be apparent to those skilled in the art that the step size X may suitably be greater or less than 200 micrometers. In particular, the step size X can be determined by a servo setting process and / or changed dynamically via drive microcode.

In step 611 monitor the servo elements 445 Servo signals on the second lateral position (output DAC = DAC + Y) of the fine actuator 420 to the servo pattern 450 to recognize. For example, monitor the servo elements 445 Servo signals by monitoring and receiving an analog signal pulse when the servo element 445 a transition of the servo pattern 450 crosses. In step 612 represents the processor 432 fixed, whether the servo elements 445 a valid servo signal according to the with respect to step 608 recognize the method described.

If the processor 432 in step 612 determines that by the servo elements 445 a valid servo signal is detected, the process goes to step 615 continued. In step 615 the processor sends 432 an output signal to the DAC to the fine actuator 420 to the first lateral position with respect to the coarse actuator 426 (eg the zero position, output DAC = 0). In addition, the processor sends in step 615 an output signal to the coarse motor 440 to the coarse-actuator 426 to move in the first direction. The process then moves to step 617 continued.

If the processor 432 in step 612 determines that by the servo elements 445 If no valid servo signal is detected, the process goes to step 614 continued.

In step 614 represents the processor 432 determines whether the limit of the travel of the fine actuator has been reached. For example, the processor provides 432 determines that a limit of the travel of the fine actuator is reached when the fine actuator 900 Micrometer has moved. As explained above, the fine actuator has a limited range of motion. If the limit of the travel of the fine actuator is not reached, the process returns to step 610 back. In step 610 becomes the fine actuator 420 by a further increment X in a first direction with respect to the coarse actuator 426 emotional.

However, if in step 614 is determined that the limit of the travel of the fine actuator has been reached, the process moves to step 616 continued. In step 616 the processor sends 432 an output signal to the DAC 434 to the fine actuator 420 to the first lateral position (eg zero position, output DAC = 0). In addition, the processor sends in step 616 an output signal to the coarse motor 440 to the coarse-actuator 426 to move in a second direction opposite to the first direction. In one embodiment, the coarse actuator may be moved anywhere in the range of 12 to 1500 microns in the second direction.

In step 617 monitor the servo elements 445 Servo signals when the coarse motor 440 the coarse actuator 426 moved in the second direction. In step 618 represents the processor 432 fixed, whether the servo elements 445 a valid servo signal according to the with respect to step 608 recognize the method described. When in step 618 a valid servo signal is detected, the process goes to step 620 continued. In step 620 the servo is locked onto the servo signal in a manner known to those skilled in the art.

If no valid servo signal is detected, the coarse actuator is moved again in the current direction (step 619 ). The current direction may be considered the immediately preceding direction of the fine actuator 420 be defined (eg the direction in which the fine actuator in step 618 has moved). The current direction may be the first direction or the second direction. If, for example, the process of step 615 comes in which the coarse actuator is currently moving in a first direction (see step 615 ), the coarse actuator moves in step 619 in the first direction. If the process, however, by step 616 comes in which the coarse actuator is currently moving in a second direction (see step 615 ), the coarse actuator moves in step 619 in the second direction. As soon as the coarse actuator is moved in the current direction, the process returns 617 back.

The steps 617 . 618 and 619 are repeated as described above until step 618 a valid servo signal is detected. Once a valid servo signal is detected within a predetermined servo lock threshold, the processor locks 432 the servo loop to the current lateral position of the tape head servo elements 445 a step 620 ). As will be apparent to those skilled in the art, the servo control unit performs 430 the normal tracking function of tracking the servo signal at the desired lateral position, which allows the reading and / or writing of data to the associated data tracks.

While the above embodiment is moving the fine actuator 420 by an increment X in discrete steps during the process of reaching the engagement on the servo signal (eg in the steps 610 . 615 . 616 and 619 ), it can be seen that the fine actuator 420 during the process can also move continuously when the servo elements 445 search for servo signals. For example, the fine actuator moves 420 during the steps 610 to 612 in a first direction until a limit of the fine actuator is reached or a valid servo signal is detected.

While in the above embodiment, the search of the servo elements 445 and the processor 432 during the process of achieving the latching on the servo signal after a valid servo signal in discrete stages (eg in the steps 606 . 612 and 618 ), it can be seen that the servo elements 445 and the processor 432 can also continuously search for a servo signal. In one embodiment, the servo elements are searching 445 and the processor 432 every 15 microseconds after the servo signal.

In the above embodiment, while moving the fine actuator in one direction with respect to the coarse actuator 426 It is important to note that in one embodiment, the coarse actuator 426 moved in the same direction in which the fine actuator 420 moves, allowing reaching the limit for the fine actuator 420 can be avoided. In one embodiment, the coarse actuator moves every 200 microseconds.

The invention may be fully embodied as a hardware embodiment or embodiment that includes both hardware and software elements for operating the magnetic tape drive 10 contains.

Furthermore, the method of the invention may be in the form of a computer program product which may be stored on a computer readable or usable medium, or which may access a computer usable or computer readable medium, the volume comprising program code for use by or in connection with a computer or any system to execute instructions. For the purposes of this specification, a computer-usable or computer-readable storage medium may be any device that includes the program for use by the system, apparatus, or unit for executing instructions or in connection with the system, apparatus, or device the unit to execute instructions or store.

The storage medium may be a system (or device or unit) of electronic, magnetic, optical, electromagnetic, infrared or semiconductor type. Examples of a computer-readable disk like 436 include a semiconductor or solid state memory, a magnetic tape, a removable computer disk, a random access memory (RAM), a read-only memory (ROM), a rigid magnetic hard disk and an optical disk. Recent examples of optical hard drives include compact disk-read only memory (CD-ROM), a compact-disk-read / write (CD-R / W) rewritable compact disk, and a DVD ,

A data processing system suitable for storing and / or executing program code includes at least one of a system bus directly or indirectly with memory elements 436 connected processor 432 ,

The present embodiment describes a method, a system and a computer program product for achieving latching on a servo signal by employing the faster moving fine servo actuator 420 for determining the location of the servo pattern 450 , The present embodiment overcomes the shortcomings of prior art methods in which the coarse actuator moved too slowly to detect the servo signal within a servo lock time threshold, causing the read and / or write to be stopped. In the present invention, the fine actuator 420 moved in a first direction until a valid servo signal is detected or the limit of the fine actuator is reached. When the limit of the fine actuator is reached, this is an indication that the servo pattern is 450 is in a second direction, and accordingly, the fine actuator 420 moved in a second direction until a valid servo signal is detected. If a valid servo signal is detected, the fine actuator becomes 420 set to zero and the coarse actuator 426 moved in the direction of the current direction (the immediately preceding direction of the fine actuator 420 , z. B. the direction in which the fine actuator in step 618 has moved). As a result, locking on the servo signal is achieved quickly and accurately.

The description of the present invention has been presented for purposes of illustration and description, but is not to be construed as exhaustive or limited to the invention in the form described. The embodiment has been chosen and described in order to best describe the principles of the invention, the practical application, and to enable others skilled in the art to understand the invention for a variety of embodiments with various modifications as appropriate for the particular particular use. It should be noted that changes may be made to the above-described method including changes to the order of steps. Further, it will be apparent to those skilled in the art that certain of the different arrangements of components illustrated herein may be used. The modifications and adaptations to these embodiments will be apparent to those skilled in the art without departing from the scope of the present invention as set forth in the following claims.

Claims (12)

A method of detecting and detecting a tracking servo pattern of a magnetic tape, the method comprising: setting a fine actuator ( 420 ) to a first position; Monitoring signals of at least one servo element ( 445 ) to detect a valid servo signal; and - in the event that no valid servo signal is detected, moving the fine actuator ( 420 ) in a first direction until either (a) a valid servo signal is detected or (b) a limit of a travel of the fine actuator ( 420 ) is reached; and - in response to reaching the limit of the travel of the fine actuator ( 420 ), Setting the fine actuator ( 420 ) to the first position and moving a coarse actuator ( 426 ) in a second direction opposite to the first direction. The method of claim 1, further comprising: in response to detecting the valid servo signal, setting the fine actuator to the first position and moving a coarse actuator in the first direction. The method of claim 1, wherein the first position is a zero position with respect to a coarse actuator. The method of claim 1, wherein a valid servo signal is within a predetermined servo lock threshold. The method of claim 4, wherein the predetermined servo lock threshold is fifty percent of servo samples. The method of claim 1, wherein moving the fine actuator ( 420 ) in the first direction moving the fine actuator ( 420 ) by an increment X. Servo system ( 400 ) for detecting and detecting a tracking servo pattern of a magnetic tape ( 21 ), wherein the servo system ( 400 ) Comprises: a tape head ( 25 ), which has a fine actuator ( 420 ) and a coarse actuator ( 426 ) and at least one servo element ( 445 ) configured to monitor servo signals; a servo control unit ( 430 ) connected to the tape head ( 25 ) for laterally positioning the tape head ( 25 ) with respect to the magnetic tape ( 21 ) connected is; and - wherein the servo control unit ( 430 ) in response to the fact that no valid servo signal is detected, capable of controlling the fine actuator ( 420 ) in a first direction until either (a) a valid servo signal is detected or (b) a limit of a travel of the fine actuator ( 420 ) is reached; and - in response to reaching the limit of the travel of the fine actuator ( 420 ) is the servo control unit ( 430 ) able to control the fine actuator ( 420 ) to the first position and a coarse actuator ( 426 ) in a second direction opposite to the first direction. The servo system of claim 7, further comprising: in response to detecting the valid servo signal, the servo controller is capable of aligning the fine actuator to the first position and moving a coarse actuator in the first direction. The servo system of claim 7, wherein the first position is a zero position with respect to a coarse actuator. Servo system ( 400 ) according to claim 7, wherein a valid servo signal is within a predetermined servo lock threshold. Sensor system ( 400 ) according to claim 10, wherein the predetermined servo lock threshold is fifty percent of servo samples. A magnetic tape storage drive comprising: a tape drive system ( 400 ) for moving a magnetic tape ( 21 ) in a longitudinal direction on the tape head ( 25 ) past; and a servo system ( 400 ) according to one of claims 7 to 11.

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